Ferrites are homogeneous ceramic materials composed of various oxides containing iron oxide as their main constituent. Being ceramic, ferrites are hard, inert and free of organic substances. Ferrites can have several distinct crystal structures, for example, magnetic soft ferrites have a cubic crystal structure.
Soft ferrites can be divided in two major categories, based upon chemical composition, manganese-zinc (MnZn) ferrite and nickel-zinc (NiZn) ferrite. In each of these categories, many different MnZn and NiZn material grades can be manufactured by varying the chemical composition or manufacturing parameters.
Soft ferrites are employed in the power transformer and chokes as used in high frequency power supplies; inductors and tuned transformers in frequency selective circuits; pulse and wide band transformers; magnetic deflection structures in TV sets and monitors; recording heads for magnetic storage devices; rotating transformers for VCR's; and shield beads and chokes for interference suppression.
Ferrites are especially useful due to a combination of high magnetic permeability which concentrates and reinforces a magnetic field and the high electrical resistivity which limits the amount of electric current flow in the ferrite. An important parameter in the application of the ferrite is the magnetic permeability. The magnetic permeability .mu. is the ratio of magnetic flux density B in Gauss to magnetic field strength H in Oersteds (.mu.=B/H).
Traditionally, magnetic permeability has been dictated by the selection and mixture of the constituent materials, the powder preparation, sintering and finishing techniques.
However, due to variations in parameters such as heating within a kiln, material impurities, moisture content of the air and ambient impurities, it is difficult to obtain a magnetic permeability for a given batch of parts within approximately 25-30% of a target value. That is, if the target magnetic permeability of a part is 10,000, the permeability generally acceptable in the market is between 7,500 and 12,500. However, for a number of applications, manual testing of the individual ferrite components is required to employ only those components having a magnetic permeability within a tighter tolerance range. The manual testing of individual parts slows the manufacturing process and increases the cost.
Therefore, the need exists for a method and apparatus for forming a ferrite, wherein the magnetic permeability may be set within a tolerance of less than 10% of a target value. The need also exists for the manufacture of ferrites without requiting extensive individual testing and processing, so that the ferrites may be economically manufactured. Further, the need exists for establishing a magnetic permeability in a ferrite which can accommodate increased tolerances in other stages of the manufacturing process.